RADEON X550 is a cheap budget card. BUT it should work correctly and without white lines.
The secondary is there, because you may connect two monitors to it. If you don't use two monitors, you can disable the secondary.: Control Panel > System > Hardware > Device manager > Display Adapters > right-click on the secondary, select "turn off' (transl?).
Please post your dxdiag report: click on following link
Diagnose.exe , select to RUN it then post the complete result into your next reply.